A method of applying coatings to threaded elements, which is particularly useful for applying fluorocarbon coatings to female threads. The method includes exploding a drop of the coating material in a solvent suspension onto the threads with an ultrasonic atomizer.
Male threaded elements, including bolts, have been coated with various coatings, including Teflon, for some time by spraying a fluid suspension of the coating material in an organic solvent on the threads or dipping the threaded portion into the coating material and then heating the threads to vaporize the solvent and cure the coating when a polymerized coating is to be formed. The waste of coating material can be reduced by spraying the threads over the source of the coating material and allowing the excess fluid suspension to drip or run off into the source. This step, of course, slows down the process and may uncover the crests of the threads. Further, conventional pressurized spraying techniques impact the sprayed area at relatively high speeds, resulting in "bounce-back", which may result in a non-uniform coating, with a thin coating on the crest of the threads and a thicker coating in the root portion. Finally, fluorinated hydrocarbons are expensive and any waste results in increased manufacturing costs.
More recently, certain automotive manufacturers have required barrier coatings of a polymerized fluorinated hydrocarbon on fasteners which are later processed through electrode deposited primers and paint. Pierce nuts and weld nuts, for example, are permanently attached to basic vehicle structural components prior to painting for interconnecting other various assembly components, including body panels. The exposed threads may become contaminated, making it more difficult to thread a bolt into the nut fastener.
Two alternative methods have been proposed for coating the threads of nut-type fasteners, including pierce nuts and weld nuts. In the first method, the coating material, which is a fluorinated hydrocarbaon such as Teflon, is suspended in an organic solvent and the suspension is sprayed on the nut threads, using a small high pressure nozzle. The fasteners are then heated to a temperature of about 450.degree. F. for 20 minutes, vaporizing the organic solvent and curing the fluorocarbon. This method has several disadvantages. First, as described above, spray coatings are uneven, with a greater accumulation of the coating in the root of the thread because the pressurized spray bounces off the crest of the threads. Second, because the fluid suspension must be relatively "dilute" to avoid clogging of the spray nozzle, the coating quickly runs off, prior to curing. Third, as described, a substantial portion of the expensive polycarbon is wasted, resulting in substantially increased material costs.
The second method includes applying the fluorocarbon in a "solid" or thick paste. This method has similar disadvantages. First, it is difficult to apply an even consistent coating to the threads. Second, the coating material is very expensive and difficult to apply, resulting in waste and increased material cost. Finally, the coating is uneven, because it is difficult to uniformly coat the root and crest of the threads.
The method of this invention eliminates many of the problems with the prior art by applying the coating with an ultrasonic atomizer which allows the use of less solvent, reduces run-off and eliminates bounce-back because the atomized coating material impacts the threads at a relatively low velocity. Finally, very little material is wasted, as described hereinbelow.